1. Field of the Invention
The present invention relates to a control for an airless spray gun having a pump piston and a pump drive arrangement including a coil and a swinging armature.
2. Description of the Prior Art
In spray guns having a pump piston driven by a swinging armature, the swinging armature coil generally is connected directly to the alternating current network. The pump piston in such spray gun executes approximately six thousand strokes per minute. For an appropriately constructed nozzle, swinging armature, pump piston, restoring spring, etc., the spray gun provides good atomization of the paint for a prescribed maximum paint throughput rate. A satisfactory degree of atomization is still obtained when the paint feed is somewhat diminished by not too great a value.
In the known spray guns, the high piston frequency of six thousand strokes per minute is also used for a diminished paint feed rate. This high piston frequency leads to wear and a considerable amount of noise during operation.
Paint spray guns are also known wherein the piston frequency is selectively reduced for use at lower paint feed rates, the frequency being reduced from six thousand strokes per minute to three thousand strokes per minute. The lowering of the stroke rate is achieved by a frequency halving by the provision of a feed and control circuit between the control network and the swinging armature coil. The feed and control circuit essentially consists of a rectifier circuit which allows only half-waves of one polarity to pass therethrough. The spray guns, however, are designed to perform at a six thousand strokes per minute rate and, therefore, satisfactory results are not achieved at the lower stroke rate. For the same nozzle size, a substantial deterioration in the quality of atomization results from halving the quantity of paint output. Moreover, an increase in the peak and effective currents as a result of the frequency halving leads to heating of the swinging armature coil.
A known spray gun includes a phase-angle control for the half-wave signals, which is utilized when the gun is switched to three thousand strokes per minute operation. The phase-angle control used during operation at three-thousand strokes per minute effectively reduces power of the apparatus and, thus, reduces the risk of overheating for the swinging armature coil. By appropriately dimensioning the size of the phase-angle control, atomization quality is satisfactory at half frequency operation.
In European Patent Application No. 0,076,510, a phase-angle control in an airless spray gun is provided for operation at both six thousand strokes per minute and three thousand strokes per minute. At least one of two phase-angle control circuits includes a variable resistor to adjust the size of the phase-angle control. This enables the degree of atomization to be varied to obtain coarser aerosol particles or even to generate what is referred to as a wet stream, such as for instance, when spraying insecticides. Since the hydraulic and mechanical elements of the spray gun are tuned for use at six thousand strokes per minute, it is not possible to reduce the minimum conveying capacity by half while still having a satisfactory atomization quality when the frequency is halved.
In none of the known airless spray guns is it possible to reduce the paint throughput while maintaining an optimum atomization, such as is possible with a compressed air spray gun. A reduced paint throughput enables even untrained persons to carry out high quality painting jobs satisfactorily.